As Papua New Guinea deals with the resignation of their Prime Minister, scientists have been exploring upheaval of a different kind - that which caused the tsunami that devastated the nation almost a year ago to day.

An international team of earth scientists have found evidence of an earthquake and underwater landslide that could be the cause of the tsunami that destroyed the Sissano Lagoon on the north coast of PNG on July 17, 1998. The three 15 metre waves killed more than 2200 people and flattened, villages, schools, churches and crops. It has resulted in a loss of oral history and relocation of residents to non-Melanesian refugee-style care centres.

Two scientific investigations earlier this year provided the first mapping of the seabed off the coast of the Sissano Lagoon looking for evidence of the cause of the magnitude 7 earthquake measured at the time of the tsunami.

A summary of the findings were delivered at the 19th Pacific Science Congress at the University of New South Wales this week by Wilfred Lus, of the Research School of Earth Sciences at the Australian National University, who represented the PNG government on the investigation.

"While we haven't quite found the smoking gun, we've found some prime candidates," Mr Lus told The Lab.

He said the team has found a number of seafloor structures that are the possible source of the tsunami. Using soundwaves and an underwater robot, they found a fault which could be the site of the earthquake and a circular slump, which could have been the site of the landslide (see picture).

The area mapped covers the offshore seabed between the Sissano Lagoon and the New Guinea Trench which marks the junction between the Australian Plate and the Pacific Plate which are colliding at a rate of 12 cm per year.

Modeling with the data collected so far has not been able to reproduce the 15 m wave height measured at the seashore, but Mr Lus said this is likely to be because the researchers were not able to get enough detail this time around. The modeling has, however, shown that these seafloor structures could produce a concentration of wave energy at the mouth of the Sissano Lagoon.

Mr Lus said that further data collection in August, would explore deeper into the slump and fault and give the researchers better detail for modeling.

He said he is particularly interested to see if there are any vertical faults around the circular slump which would support the hypothesis that the tsunami was caused by reactivation of an old slump by the earthquake.

Mr Lus says that the area investigated is clearly unstable with evidence of recent movement so it is uncertain if the Sissano Lagoon area would be safe for resettlement.

He said it would be better if people could rebuild their houses at least a kilometre away from the coastline although he acknowledged this would have a major impact on the lifestyle of residents.

The investigations were a joint project of the Japan Marine Science and Technology Centre (JAMSTECH) and the South Pacific Applied Geoscience Commission (SOPAC) and the findings will be published in the Eos the journal of the American Geophysical Union.